1. Field of the Invention
This invention relates to an erect type zoom telescopic optical system for a binocular or monocular telescope having objective lens system, eye-piece lens system and erect prism for erecting an image.
2. Description of the Prior Art
Telescopes for observing a space image formed by an objective lens system through an eye-piece lens system is known as Kepler type and binocular or monocular telescope of this type has been used for a long time. In binocular telescope two optical systems are arranged in parellel and these two optical systems are equivalent except diopter correction for left or right eyes, respectively. Monocular telescope consists of one of these two optical systems so that in the following we use simply the term binocular to cover both binocular and monocular telescopes.
In such binocular, there has been already known zoom binocular which enables to continuously change magnification. In case of so-called eye-piece zoom in which magnification of eye-piece lens system is changed, it is possible to obtain a relatively simple structure as aperture of eye-piece itself is small. Owing to the face that magnification is varied within eye-piece lens system having a short focal length, however, it is extremely difficult to correct aberrations through a wide range of field of view while maintaining eye relief, which is a distance between eye point and an exit light side surface of eye-piece lens, at a predetermined length. As a result, it is only realized for a narrow range of field. Further, in general, since field stop is fixed, apparent field of view is changed as magnification varies. Especially, in middle type eye-piece zoom; which is called as eye-piece-objective zoom, in which a space image is formed by objective within eye-piece, the field stop does not sufficiently play a role as a stop due to the fact that lenses sandwiching field stop are moved and so image is moved relative to fixed stop. This problem can be solved by interlocking the stop to the movement of lenses at the time of zooming operation and varying the aperture size of the stop, however, this requires a complex structure.
On the other hand, in case of varying the magnification by objective lens, which is called as objective zooming, the whole system becomes bulky and complex as the zooming operation is performed by the movement of the objective having a large aperture. For making the size of the zooming system of the objective small, the refractive power of each lens must be large, which result in the deterioration of aberrations and practical small size zoom binocular could not be attained.